Contact imaging sensor head for computed radiography

ABSTRACT

A scan head design uses 1:1 (one-to-one) imaging micro-lens arrays to transfer the object plane X-ray image from a CR-plate onto a linear photosensor. The scan-head includes a housing having therein, an array of red light emitting diodes (LEDs), a red-absorbing filter, a microlens array, an infrared-filter, and a sensor. The housing faces the CR-plate and the scan-head is translated across the CR-plate to read out the X-ray image therein. The scan head is compact and provides for improved spatial resolution and reduced power requirements.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.15/622,939, filed Jun. 14, 2017, titled “Contact Imaging Sensor Head ForComputed Radiography,”, which claims benefit of priority to U.S.Provisional Patent Application No. 62/351,390, entitled “Contact ImagingSensor Head for Computed Radiography,” filed Jun. 17, 2016, both ofwhich are incorporated herein by reference in their entirety.

FIELD OF THE EMBODIMENTS

The purpose of the invention is to provide an improved linear imagingsensor assembly which may be used as X-ray imaging diagnostics formilitary and first-responders working in explosive ordinance disposal(EOD) and improvised explosive device (IED) investigation and disposal.

BACKGROUND

Computed Radiography (CR) involves readout-scanning of a photostimulablestorage phosphor plate (CR-plate) previously exposed to an X-ray sourcein an imaging mode. The technology was developed as an alternative tofilm-based radiography and has been used for decades in the medical andveterinary areas. The medical readout devices use flying-spot laserscanners with large and complex optical assemblies in bulky chassisdesigned for an office environment. These devices are not suitable forthe man-portable, field applications intended for computed radiographyproducts such as the Future Radiography System (FRS) and the Real TimeRadiography (RTR) system products developed by Applicant, Leidos, Inc.

The Leidos FRS products are the only known CR systems compatible withfield operations that also meet the form-factor requirements of themilitary and first-responder customers. The limiting form factors arethe overall height (thickness) of the cassette/reader system, about 1.5inch or less, plus the requirement for minimum dead space on two sidesat a corner.

An existing Leidos product is based on a scan head described in U.S.Pat. No. 7,825,394 (hereafter the '394 patent”), the substance of whichis incorporated herein by reference in its entirety. The prior art scanhead described in the referenced patent uses a linear array ofcharge-coupled device (CCD) photosensors placed very close to the CRplate to enable contact-transfer (sometimes referred to as proximityfocusing) of the violet, stimulated light to the readout pixels. Thereare no imaging optics between the CR plate surface and the CCDarray—only a thin filter and a protection film. The filter is requiredto eliminate the red stimulation light from the violet CR plate emissionand is approximately 250 microns thick (est.). The protection film,typically 60-70 micron thick Mylar, protects the scan head, reducesdamage to the fragile CR plate and provides a smooth-running interface.The total height of the head assembly is about 10 mm which is quitecompact and helps meet the overall thickness goal. The width of the headis also about 10 mm although this dimension is less critical as long asthe design supports the minimum dead-space requirement at one edge. Theactive length of the head is 14 inches with only a few mm of dead spaceon each end for coupling to the mechanical scan drive.

Red-light stimulation of the CR plate is accomplished using a densearray of LEDs aimed at the plate along the length of the prior art head.The LEDs are separated from the CCD array by a thin metal wall extendingall the way down to the CR plate surface. This “knife-edge” interfaceresults in thin line-source of red light that is created byscatter-diffusion through the 290 micron thick CR plate around to theregion viewed by the CCD array. An advantage of this simple geometry isthat the CCD array can be positioned directly against the knife-edgewall ensuring good alignment relative to the line-source emission. Anadditional advantage is that the flood-illuminated region directly underthe LEDs affords effective erasure of the CR plate during the primaryscan and also during the return-to-home of the head, if the LEDs remainenergized. Obviously, the head can run the primary scan in only onedirection.

The system runs in a pushbroom-style scan imaging mode taking about 25seconds to travel the 17-inch length of the CR plate. The image isdivided into square pixels 172 microns on a side. There are 2400 pixelsin the 17-inch direction (direction of mechanical translation in FRS).Hence, the read-integration time for each pixel is about 10.5 ms. TheLED power must be adjusted to bleach out more than 90% of the storedviolet photons in this time interval, but not too high such thatadditional bleaching occurs outside the photosensitive collection regionin the scan direction. Other than this requirement, the only limit onscan speed is the readout time of the CCD array and the capabilities ofthe mechanical drive system.

By way of example only, the CR plates are very high quality and may besupplied by a manufacturer such as Carestream Health, Inc. The CR plateis a highly diffusive granular phosphor material that is flat, smooth,and appears bright-white in room lights. The current FRS system uses theCarestream General Purpose or GP plate providing the highest X-raystopping power.

SUMMARY OF THE EMBODIMENTS

In a first exemplary embodiment, a system for readout-scanning of apreviously exposed photostimulable storage phosphor plate containing anX-ray image includes: a linear array of red light emitting diodes (LEDs)oriented at a predetermined angle to the exposed photostimulable storagephosphor plate to illuminate a gap-interface between the exposedphotostimulable storage phosphor plate and a red-absorbing filter; and amicrolens array for receiving image photons emitted from the exposedphotostimulable storage phosphor plate responsive to the illuminationfrom the linear array of red light emitting diodes (LEDs) and passedthrough the red-absorbing filter, wherein the microlens array forms theX-ray image on a linear sensor.

In a second exemplary embodiment, a scan-head for readout-scanning of apreviously exposed photostimulable storage phosphor plate containing anX-ray image includes: a housing including therein, an array of red lightemitting diodes (LEDs), a red-absorbing filter, a microlens array, aninfrared-filter, and a sensor; a sheet of cover glass adhered to a firstportion of a side of the housing facing the exposed photostimulablestorage phosphor plate; and multiple pads adhered to at least a secondand third portion of the side of the housing facing the exposedphotostimulable storage phosphor plate; wherein the scan-head istranslatable across the previously exposed photostimulable storagephosphor plate to read out the X-ray image therein.

In a third exemplary embodiment, subsystem for directing a read-out beamto a CR-plate to stimulate emission of image X-ray photons therefrom,includes: an array of red light emitting diodes (LEDs) for emittingread-out light; and a red-absorbing filter, wherein the array of redLEDs is oriented at a first predetermined angle to the CR-plate toilluminate a gap-interface between the CR-plate and a parallel side ofthe red-absorbing filter; and further wherein the red-absorbing filterincludes a beveled edge facing in a direction of the linear array of redlight emitting diodes (LEDs) and forming a second predetermined with afacing side of the CR-plate, such that the read-out beam and the bevelededge operate to produce the stimulated emission line-source of imageX-ray photons.

BRIEF DESCRIPTION OF THE FIGURES

The following Figures are to be considered in conjunction with thedetailed description below:

FIGS. 1(a) and 1(b) are cross-sectional, schematic views of a scan headdesign in accordance with a first embodiment described herein;

FIG. 2 is an illumination geometry as modeled with ray tracing;

FIG. 3(a) is an images of line pair gauges from the prior art;

FIG. 3(b) is an images of line pair gauges from the first embodimentdescribed herein;

FIG. 4 is a cross-sectional, schematic view of a scan head design inaccordance with a second embodiment described herein.

DESCRIPTION OF THE EMBODIMENTS

FIGS. 1(a) and 1(b) show a cross sectional view of a first embodiment ofthe present scan head design which uses 1:1 (one-to-one) imagingmicro-lens arrays to transfer the object plane image onto a linearphotosensor. This form of imaging should not be confused with proximityfocusing as described above in the background.

The embodiment of FIGS. 1(a) and 1(b) consists of a red LED linear arrayor light strip (a) oriented to illuminate a gap-interface (G) betweenthe CR plate (b) and a red-absorbing filter (c), e.g., Schott BG3, thatpasses the image photons emitted from the narrow line-source object (d)to a Selfoc Lens Array (SLA) (e) which forms the image at the CMOSlinear sensor (f). In this exemplary embodiment an additionalinfrared-absorbing filter (g), e.g., Schott S182 or intereference-typefilter, is used to eliminate the IR component of the LED light thatleaks through the red-absorbing filter. A thin glass cover plate (h)bonded to the filter (c) and to the housing (H) seals the optics andCMOS sensor from moisture and contamination. Thin Teflon pads (i)elevate the scan head to prevent direct contact between the glass coverplate (h) and the CR plate (b). The pads (i) ensure smooth, low abrasionmotion of the scan head while in contact with the CR plate. Positivepressure between the CR plate and the scan head is maintained by aspring and roller assembly incorporated into the cassette/reader design.The approximately 24 mm wide housing is representative of the prototypebut the overall design can potentially be modified to be 18 mm to 20 mmin width.

In the embodiments herein, an exemplary baseline SLA is a model 20DG2-row commercial off the shelf (COTS) product manufactured by GoFoton, asubsidiary of Nippon Sheet Glass, Inc. This lens array projects a 1:1erect, real image onto the CMOS linear sensor. One skilled in the artrecognizes that equivalent lens arrays are contemplated by thisdescription and are considered to be within the scope of theembodiments.

The red-absorbing filter has a 45° angle-cut edge facing the LED array.The angle-cut is required to accommodate the ±25° acceptance angle ofthe 2-row SLA viewing the emission-line object and also directs the LEDlight across the interface gap. All distances and focusing properties ofthe system have been determined and optimized by 3-D optical raytracemodeling taking into account the emission spectrum and thedispersive/absorptive properties of the filters and lens material. Theseresults show an optical spatial resolution of better than 6 lp/mm (linepairs per millimeter).

The surface of the beveled edge (j) is mirror-coated with aluminum orsimilar reflective metal to prevent the intense, direct LED light fromentering the short-pass filter (c). All other optical pathways into theshort-pass filter are blocked or sealed except for the front surfacefacing the CR plate. In the baseline embodiment, we have chosenthin-film aluminum applied by vacuum deposition. This choice providesthe required isolation and also creates a mirror reflector that enhancesthe irradiation of the LED flood zone facilitating erasure of the CRplate. A less reflective coating, up to and including flat black, on thebeveled edge surface (j) is also an option and may be selected tocontrol the optical flux distribution. Thin metal shims may also besubstituted for the coating.

The angle-cut, coated surface (j) acts as a baffle to cut off the LEDlight entering the gap between CR plate and the front surface of thered-absorbing filter (c) producing a sharp boundary in the red lightimpinging on the CR plate. The gap can be adjusted to provide anirradiated region on the CR plate surface with a width approximatelyequal to G/tan (θ), where G is the gap distance and θ is the LED anglerelative to the CR plate. We refer to this geometry as an “optical fluxpartition” describing the baffle-plus-gap functional geometry. Diffusionof red light from the LED flood zone into the gap region via bulkscattering in the CR plate is of secondary importance compared to thisdirect irradiation component.

The CR plate violet light emission in the band approximately 350 nm to450 nm is stimulated by the irradiated region distributed over the CRplate. Approximately 0.3 mJ/cm² of incident red light (nominal centerwavelength of 600 nm) is required to liberate about 95% of the storedviolet light representing the X-ray image. The LED flux is accumulatedduring an integration time determined by the pushbroom-scan time neededto form square pixels in the digital image. In the prior art FRS productand in the present embodiment, that integration time is approximately 10ms. Very intense LED light sources are required to provide irradiation≥0.3 mJ/cm² in ≤10 ms while maintaining a high degree of uniformity.

This level of irradiation can be best achieved in our geometry using ahigh-density (high number per unit length) linear LED array placed a fewmillimeters from the CR plate/gap interface. The red-light intensity atthe interface is further increased by selecting surface-mount LEDdevices having small, dome lenses that focus the light in the forwarddirection. The embodied design is based on placing these LEDs at aspacing (pitch) of approximately 1.0 mm. The geometry is shown in FIGS.1(a) and 1(b). We have determined, through ray trace simulations anddirect measurements, that this LED array can deliver the requiredirradiation operating at about 40% of its maximum power. The intensityuniformity at the gap interface is better than two percent as predictedby ray tracing results and by direct measurements in the laboratory.

FIG. 2 shows an example of the illumination geometry as modeled in ourray tracing simulations. Our optical ray tracing simulations are carriedout using the commercial engineering code TracePro® sold by LambdaResearch, Inc. TracePro provides advanced tools for analysis of imagingsystems and lighting conditions including utilities for exact modelingof LED components based on vendor data specifications. In this case weshow the results for an 80 micron air gap (G) that produces a surfaceirradiation pattern approximately 130 microns in width. The gap width(G) can be adjusted larger or smaller in the final design. As in theprior art FRS design, image scanning must be unidirectional. Byinspection one can see that most of the LED emission impinges on the CRplate downstream of the imaging interface providing for robust erasureof plate in the scan direction and in the return of the scan head to thehome position (assuming the LED array remains energized).

Form Factor

From FIG. 1(b), it is clear that this embodiment of the invention meetsthe requirement for a scan head height, i.e., 13 mm, that fits within a1.5″ cassette (or approximately 38 mm). One skilled in the artrecognizes that the present embodiment can be modified to accommodatelengths of 8″-17″ across the full range of anticipated scanner productdimensions.

Operational Characteristics

Various COTS components may be incorporated in the embodiments inaccordance with required environmental extremes and are expected tooutperform the prior art systems in this respect. This includes the SLAand CMOS array. The optical design has been developed to meet expectedenvironmental and ruggedness requirements. The LED illumination systemhas been selected to enable pixel-integration times of less than 10 msso as to maintain read times of less than 30 seconds. The LED array ismuch more efficient at stimulating blue emission and therefore requiresonly about one third the electrical power compared to the LED array inthe prior art scan head.

Readout electronics for CMOS line scanners are much simpler than forCCDs since the charge-shifting and transimpedance amplification is alltaken care of on the silicon adjacent to the photodiode pixel. All thatis needed is a clock, a frame start-pulse (determines the integrationtime) and an A/D converter. There are various design options for formingand reading out the image data but the board-level computer currentlyrequired in the prior-art FRS can be eliminated, reducing cost, loweringthe power draw and freeing up space behind the CR plate bed. The extraspace will enable a new cassette design having a thickness less than thecurrent 1.5″.

Higher Performance

Spatial resolution in the proximity-focus design of the prior art headis limited by the approximately 350 micron separation distance betweenthe CR plate and CCD array. The prior art system has a spatialresolution of less than 2 lp/mm, on average. The embodiment describedand illustrated herein has an optical resolution better than 6 lp/mm.Spatial resolution is primarily limited by the CR plate characteristicsand pixel size. In a first exemplary embodiment using 125 micron pixelsa spatial resolution of approximately of 2.75-3.0 lp/mm in allorientations has been achieved. We have verified this result inprototype testing using the standard Carestream GP plate. Even higherresolution is possible with thinner CR plates and custom pixel sizes.FIGS. 3(a) and 3(b) present images of line pair gauges from the priorart system (FIG. 3(a)) and from the exemplary embodiment (FIG.3(b))—taken at equivalent scan speeds. The prior art FRS image in FIG.3(a) illustrates the worst-case orientation: in the direction of thesensor. Clearly the spatial resolution is no better than about 1.6 lp/mmin this case. The spatial resolution example from the exemplaryembodiment FIG. 3(b) was obtained under realistic operating conditions.

In a second embodiment shown in FIG. 4, the CR plate is viewed by a2-row Selfoc Lens Array (SLA) projecting a 1:1 erect real image onto aCMOS linear sensor. At the interface of the SLA and the CR is aninfrared (IR) filter with a beveled edge having a thick (500 nm)aluminized coating coupled to a blue/violet short pass filter on theopposite side with a mating bevel. The mirrored surface directs LEDlight from a red LED array onto the CR plate at precisely the centerlineof the SLA field of view. This glass/mirror edge defines a line sourceon the CR plate by way of diffusion of the red light through the 290micron thick phosphor matrix across the edge-tip into the field of viewof the SLA. The emission of violet light (350-450 nm) overlays theresulting red light line-source as distributed in the CR plate.Approximately 0.3 mJ/cm² of red light (550-650 nm) is required toliberate 95% of the stored violet light representing the X-ray image.

Measurements indicate the half-width of the glass/mirror line source tobe on the order of 75 microns. Using a single row CMOS photosensor array(1×N pixels) dictates that the pixel size be larger than the line sourceto provide efficient collection of the violet light. We have developed acustom CMOS array having 125 micron wide by 200 micron long pixels. The200 micron length, exceeding the full width of the imaged line source,is intended to facilitate final alignment of the optical system whileminimizing loss of signal and maintaining full spatial resolution in allorientations. The fill factor, dark current, read noise and sensitivityparameters of the custom CMOS device were selected to achieve thesignal-to-noise goals of the operating system which will equal or exceedthat of the prior art system.

The glass/mirror optical flux partition and filter combination interfaceof the embodiments is superior to the plastic tape, metal wall andepoxy/dye-filter interface of the prior art scan head. Fabricated usingoptical CNC (computer numerical control) technology, this embodimentprovides extremely precise (e.g., ±55 micron) alignment of the violetline-source emission with the optical axis of the SLA over the fulllength of the scan head. The embodiment described herein stimulates theCR plate emission via direct illumination controlled by the fluxpartition, LED and gap geometry, whereas the prior art design reliesupon diffusion of the red LED light through the CR plate bulk materialpast the metal-foil knife edge. Since diffusion is a second order effectcompared to the direct surface illumination, the present embodiments areable to operate at about one third the LED power of the prior art scanhead.

The foregoing discussion discloses and describes merely exemplaryembodiments of the technology described herein. One skilled in the artwill readily recognize from such discussion and from the accompanyingdrawings that various changes, modifications and variations can be madetherein without departing from the spirit and scope of the invention.

The invention claimed is:
 1. A system for readout-scanning of apreviously exposed photostimulable storage phosphor plate containing anX-ray image comprising: an array of red light emitting diodes (LEDs)oriented at a predetermined angle to the exposed photostimulable storagephosphor plate to illuminate directly both a gap-interface between theexposed photostimulable storage phosphor plate and a red-absorbingfilter and a first predetermined portion of the exposed photostimulablestorage phosphor plate; and an imaging array for receiving image photonsemitted from the exposed first predetermined portion of thephotostimulable storage phosphor plate responsive to the illuminationfrom the array of red light emitting diodes (LEDs) and passed throughthe red-absorbing filter, wherein the imaging array forms the X-rayimage on a sensor.
 2. The system of claim 1, further comprising: an IRfilter located between the imaging array and the sensor.
 3. The systemof claim 1, wherein the predetermined angle is less than 45 degrees. 4.The system of claim 1, wherein the red-absorbing filter includes abeveled edge facing in a direction of the array of red light emittingdiodes (LEDs).
 5. The system of claim 4, wherein the beveled edge of thered-absorbing filter includes a reflective coating.
 6. The system ofclaim 1, wherein the sensor is a CMOS sensor.
 7. The system of claim 1,wherein the imaging array is a microlens array.
 8. The system of claim1, wherein the system height is equal to or less than 13 mm.
 9. Thesystem of claim 1, wherein the array of red light emitting diodes (LEDs)comprises surface-mount LED devices with individual dome lenses forfocusing LED emitted light in a forward direction.
 10. The system ofclaim 9, wherein the LEDs have a spacing of approximately 1.0 mm. 11.The system of claim 9, wherein the system focuses the LED emitted lightat a line on the exposed photostimulable storage phosphor plateresulting in a stimulated emission line-source of image photonstherefrom.
 12. A scan-head for readout-scanning of a previously exposedphotostimulable storage phosphor plate containing an X-ray imagecomprising: a housing including therein, an array of red light emittingdiodes (LEDs), a red-absorbing filter including a beveled edge facing ina direction of the array of red light emitting diodes wherein thebeveled edge includes a reflective coating, an imaging array, aninfrared-filter, and a sensor; wherein the scan-head is translatableacross the previously exposed photostimulable storage phosphor plate toread out the X-ray image therein; and further wherein the array of redlight emitting diodes is oriented at a predetermined angle to theexposed photostimulable storage phosphor plate to illuminate directly afirst predetermined portion of the exposed photostimulable storagephosphor plate; and further wherein the imaging array receives imagephotons emitted from the exposed first predetermined portion of thephotostimulable storage phosphor plate responsive to the illuminationfrom the array of red light emitting diodes (LEDs) after passing throughthe red-absorbing filter, and forms the X-ray image on the sensor fromat least the first predetermined portion of the exposed photostimulablestorage phosphor plate.
 13. The scan-head according to claim 12, whereinthe housing has dimensions equal to or less than a height of 13 mm and awidth of 24 mm.
 14. The scan-head of claim 12, wherein the array of redLEDs illuminates a gap-interface between the exposed photostimulablestorage phosphor plate and the red-absorbing filter.
 15. The scan-headof claim 14, wherein the predetermined angle is less than 45 degrees.16. The scan-head of claim 12, further comprising: a sheet of coverglass adhered to a first portion of a side of the housing facing theexposed photostimulable storage phosphor plate; and multiple padsadhered to at least a second and third portion of the side of thehousing facing the exposed photostimulable storage phosphor plate. 17.The scan-head of claim 12, wherein the sensor is a CMOS sensor.
 18. Thescan-head of claim 12, wherein the imaging array is a mircolens array.19. The scan-head of claim 12, wherein the array of red light emittingdiodes comprises surface-mount LED devices with individual dome lensesfor focusing LED emitted light in a forward direction.
 20. The scan-headof claim 19, wherein the LEDs have a spacing of approximately 1.0 mm.21. The scan-head of claim 19, wherein the scan-head focuses the LEDemitted light at a line on the exposed photostimulable storage phosphorplate resulting in a stimulated emission line-source of image photonstherefrom.